The present invention relates to a process for casting thermosetting resins, specifically to a casting process useful for manufacturing laminated ophthalmic lenses containing allyl diglycol carbonate thermosetting resins.
Diethylene glycol bis(allyl carbonate) resin, a specific thermosetting resin commercially available as CR-39.RTM. resin, is presently used to produce finished and semi-finished ophthalmic lenses by casting processes. The molds presently used for casting such lenses include glass components having optically finished resin contact surfaces which contain the resin during curing and form the front and/or back surfaces of the cast lens.
Mold properties which are important for this process include mold strength, thermal expansion, and thermal conductivity, and also the finishing and resin parting characteristics of the mold. Good thermal conductivity is required so that the mold will readily transmit heat to and from the resin during the curing cycle. The mold material must have a low thermal expansion coefficient so that the mold curvature during curing at elevated temperatures will not differ significantly from its curvature at room temperatures.
In order to realize useful mold life, the strength of the mold must be high and the forces necessary to part the mold from the cured resin must be relatively low. Otherwise mold breakage will occur and, in the case of excess adherence between the resin and mold, low lens selection rates due to lens cracking and/or surface defects are encountered.
On the other hand, some minimum adherence between the curing resin and the optical surfaces of the mold is required. A mold which "self-releases" during curing produces a lens which is out of curvature, or which exhibits a "milky" or mottled surface. It is necessary that the surfaces of the cured lens accurately reproduce the optical surface quality of the mold if a subsequent lens polishing step is to be avoided.
Glass-ceramics are relatively new materials exhibiting properties which are in certain respects superior to those of glasses. See, for example, Glass, Its Industrial Applications, C. J. Phillips, Reinhold, New York (1960), pp. 191-195. These materials have been used for forming softened glass in the glass industry, as suggested in U.S. Pat. No. 4,052,184 to Anderson, and have been proposed for use in making injection mold inserts for molding thermoplastic resins at elevated temperatures and pressures as in German OLS No. 2,204,830.
Notwithstanding these facts, the mold material most widely used at present for the casting of finished plastic lenses is white crown glass. Typical properties for such glass are an average linear thermal expansion coefficient (0.degree.-300.degree. C.) of about 96.times.10.sup.-7 /.degree.C., a room temperature thermal conductivity of about 0.0024 cal/cm. sec..degree.C., and a modulus of rupture strength of about 7,000 psi.
Glasses exhibit the necessary adherence to allyl digycol carbonate resins during curing so that self-release leading to poor surface quality and/or out-of-curvature lenses is largely avoided. Also, glass molds are relatively easily parted from cured plastic lenses, provided that the current industry practice of separating the lenses from the molds at relatively high temperatures, e.g., 60.degree. C. or above, is followed.
Recently it has been proposed to provide light weight glass-plastic laminated lenses comprising bonded thin glass and allyl diglycol carbonate laminae by direct casting of the liquid monomer into molds containing thin elements of sheet glass. Hence my copending, commonly assigned application, Ser. No. 27,231, filed Apr. 5, 1979, describes the production of laminated glass-plastic lenses by a direct casting technique. However, it has been found that such laminated lenses cannot be economically manufactured if removed from the molds at the high mold separation temperatures commonly employed in the plastic lens manufacturing industry. High temperature separation has been found to lead to a high incidence of stress failure on subsequent testing of laminated lenses produced in this manner.
On the other hand, it has been found that if plastic lenses are cooled to relatively low temperatures (e.g., room temperature) in contact with glass molds, separation of the plastic from the mold is quite difficult, and cracking of the plastic during the removal of the glass mold frequently results.
It is a principal object of the present invention to provide a method for producing glass-plastic laminated lenses economically through the use of a new mold material and manufacturing process.
It is a further object of the invention to provide a manufacturing process offering higher selection rates and a broader range of permissible lens design parameters than prior art processes.
Other objects and advantages of the invention will become apparent from the following detailed description thereof.